Forest habitat invasions – Who with whom, where and why

Abstract Even though temperate forests have lower non-native plant species richness and cover in comparison to some other habitats, such as anthropogenically influenced habitats or some grassland habitats, several recent studies from Central Europe suggest that there has been an increase in the numbers and proportions of non-native species in forests. Previous studies on the level of invasion in forest habitats were usually conducted on a relatively coarse scale, often excluding plantations of non-native trees or analyzing them together as a group regardless of the different biological properties of different species. Therefore, a more detailed analysis would be helpful to obtain better insight into the processes taking place in forests, focusing not only on the analysis of the invasion level of different forest types at a finer scale (e.g., in various types of floodplain, beech or coniferous) but also on the character of the non-native plants found there (invasive status, frequencies and habitat preferences). In this study, we aimed to (i) compare the invasion level of various forest habitats (including forests dominated by non-native trees) in Slovakia with regard to different residence times and invasive status of non-native species, (ii) analyze whether forest habitats possess specific non-native species combinations, and (iii) determine the preferences of non-native plants with respect to specific forest habitats and the co-occurrence of other plant species with the most frequent non-native species. Our study confirmed that almost all forest habitats contained at least one non-native species. Native forests at lower elevations and closer to rivers were more invaded, while broadleaved, mixed and especially coniferous forests at higher elevations and ecologically strictly limited forests were less (or un-) invaded. Significant differences between various types of riparian and coniferous forests, emphasized the importance of the more detailed discrimination of the forest habitats in comparison with previous studies. Non-native tree plantations, which are among the most invaded forests, also deserve special attention. Some non-native trees form stable communities with typical native and non-native species, where non-native dominants can act as drivers of further changes in the vegetation composition, facilitating spread of other co-occurring non-native shrubs and herbs. Taking into consideration the potential negative impacts of some non-native plants on species composition, forest microclimate and soil chemistry, it would be advisable for foresters to factor this into forest management because their choice of planted species may both positively and negatively influence the level of invasion in managed forests.